Dimmer circuit for gas discharge tubes



May 23, 196 J. R. GARNETT ETAL 3,321,666

DIMMER CIRCUIT FOR GAS DISCHARGE TUBES Filed Dec. 21, 1964 CONTROLCIRCUITS I INVENTORS JAMES R. GARNETT HARRY ROWN 3 1 3 BY A T TORNEYSUnited States Patent 3,321,666 DIMMER CIRCUIT FOR GAS DISCHARGE TUBESJames R. Garnett and Harry Brown, both of Philadelphia,

Pa., assignors to the United States of America as represeuted by theSecretary of the Navy Filed Dec. 21, 1964, Ser. No. 420,221 3 Claims.(Cl. 315-269) The invention described herein may be manufactured andused by or for the Government of the United States of America forgovernmental purposes without the payment of any royalties thereon ortherefor.

The present invention relates to a brightness control circuit and moreparticularly to a dimmer circuit for a gas discharge tube and the like.

In the field of instrumentation it has been the general practice toemploy light tubes of various kinds as means for signalling thecondition of various instruments. A significant example is thewell-known nixie tube which is a gas discharge device containing numbersfrom 0 to 9 or letters or other indications. The tube is capable oflighting any one of these numbers or letters as desired according to thecondition of the instrument. The individual figures in the tube are gasdischarge devices which require a voltage above a specified minimum inorder to light. A difiiculty has arisen in that in certaincircumstances, such as, for example, in the low light level of anairplane cockpit the light emanating from the nixie tube or other deviceis too bright and is annoying and distracting to the operator. Standardmeans for dimming such devices has simply consisted of lowering thevoltage across the tube. However, since the tubes must have a specifiedminimum voltage in order to light, it has been found that lowering thevoltage across the tube quickly results in impairment of the function ofthe tube and failure of the tube when the voltage goes below thespecified minimum.

The general purpose of this invention is to provide a circuit by whichthe amount of power fed into the tube may be lowered without loweringthe peak voltage put across the tube, so that the net light eifect ofthe tube is lessened without failure. To attain this, the presentinvention provides a circuit having an oscillator connected to avariable resistance element whereby the circuit alternately has a veryhigh and very low resistance in series with the tube. The result of thisis to provide to the tube a series of oscillations comprising peaks ofsufficient voltage to keep the tube 11 lit, in which the amount of powerfed to the tube is a function of the percentage of the cycle duringwhich the peak voltage is on, which percentage is controlled by theoscillator. The resistance element may be a transistor used as anamplifier, or a relay. The control of the oscillator is provided byvariable resistances contained in the oscillator which control thefrequency of the oscillator or the percentage of the oscillator cycle inwhich the condition of the oscillator is sufticient to turn off thetransistor or relay.

Accordingly, it is an object ofthe present invention to provide a dimmercircuit for a gas discharge tube such as a nixie tube, whereby the peakvoltage of the circuit is above the minimum voltage of the tube but theamount of power fed in is a function of a variable device controlled bythe operator.

Another object of the invention is to provide an oscillator for avariable resistance element connected to a gas discharge tube to controlthe power fed into the gas discharge tube.

A further object of the invention is the provision of control meanswithin the oscillator to control the On-Off ratio of the oscillatorcycle.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detiled description when considered inconnection with the accompanying drawings in which like referencenurnerals designate like parts throughout the figures thereof andwherein:

FIG. 1 shows a circuit diagram of an oscillator, resistance element, andgas discharge tubes according to the present invention.

FIG. 2 shows a voltage-current diagram of an oscillator element in theoscillator of FIG. 1.

FIG. 3 shows the variation with time of the voltage signal on theoscillator of FIG. 1.

FIG. 4 shows an alternative embodiment of the resistance element in FIG.1.

In FIG. 1 there is shown a set 11 of gas discharge tubes numbered in thedrawing from 1 through 0. These numbers represent the components of agas discharge tube used for readouts such as a nixie tube and all tennumbers will generally be contained within a single tube. A set ofcontrol circuits 12 is connected to the ten numbers in the tube andcontrols the application of a voltage to the tubes as shown. All of thetubes are connected at their opposite sides in parallel to a resistanceelement 13 which is shown as a transistor in FIG. 1. The-base oftransistor 13 is connected by a pair of resistances 14 and 15 to avoltage source. Connected between the junction of resistances 14 and 15and ground is a second transistor 16 controlling the signal on the baseof transistor 13. Connected to the base of transistor 16 is a resistance17 which as shown may be variable. Variable resistance 17 is connectedto the voltage source through another resistance 18 which as shown mayalso be variable.

The operation of the oscillator is conducted through a unijunctiontransistor 21 the poles or bases of which are connected between thevoltage source and ground 'by a pair of resistances 22, 23. Acapacitance 24 is connected between the emitter of transistor 21 andground.

The operation of the relaxation oscillator is as follows. As shown inFIG. 2, the characteristic of unijunction transistor 21 is that itscurrent increases with voltage up to a specified point, identified as Von the base thereof. When the voltage on the base of transistor 21 as.measured at the junction V in FIG. 1 attains the value of V the currentwill suddenly increase into the negative resistance section oftransistor 21, as shown in FIG. 2, which will cause the charge oncapacitor 24 to discharge into transistor 21 through the emitterthereof. This discharge also causes the resistance across the poles oftransistor 21 to decrease greatly, and voltage thereacross is greatlyreduced. The elfect of this is to cause the condition of transistor 21to return to the positive resistance side of the curve in FIG. 2 and thecycle is repeated. The frequency of the cycle is determined by the timethat it takes charge to build up on capacitor 24 to a voltage V Thistime is determined mainly by the resistance of variable resistance 18.If variable resistance 18 is high, the frequency will be low and theperiod to charge up will be high. If resistance 18 is very low,capacitor 24 will charge up quickly and the frequency of the oscillatorwill be high. FIGURE 3 shows the relationship of the voltage at junctionV with respect to the time on and time olf. During time on, transistoror resistance element 13 has an essentially zero resistance and the tube11 is on. During the time 011, the resistance of element 13 isessentially infinite and tube 11 is oil. If resistance element 13 is atransistor, the transition from zero to infinite resistance will not beinstantaneous but the transition is quite quick and for practicalpurposes considering the range of voltages involved in FIG. 3 it may beconsidered is substantially instantaneous at voltage V If the resistance18 is decreased, it will increase the rate of charging up of capacitor24 and the base of transistor 21 will arrive at V quicker. The result ofthis will be to cause transistor 16 to be conducting a major part of thetime. When transistor 16 is conducting, the connection to the base oftransistor 13 is essentially shorted and it is off. Thus decreasingresistance 18 increases the time during which transistor 13 is blockingtube 11 and thereby decreases the brightness. Increasing resistance 18causes the curve in FIG. 3 to be longer, lowers the frequency and causesa greater percentage of the time of each cycle to be in the On stage.Therefore increasing resistance 18 causes the tube 11 to be brighter. Ifresistance 17 is varied, this varies the proportion of voltage V whichis placed across the base of transistor 16. Resistance 17 and the inputresistance of transistor 16 form a voltage divider. The effect ofraising resistance 17 is to place a lower percentage of the voltage atjunction V on the base of the transistor 16 and thereby to causejunction V to charge up to a higher voltage before conduction oftransistor 16 takes place. By this means a greater percentage of thecycle time will be in the On stage, and increasing the resistance 17increases the brightness of the tube 11. The effect of increasingresistance 17 may be seen in FIG. 3 if it is considered that theeffective voltage V, is raised toward the voltage V which, as will beseen, will cause a greater percentage of the time to be shifted into theT section. In FIG. 3, when the voltage is between V and V the tube 11 ison and when the voltage between V and V the tube 11 is off.

In FIG. 4 is shown an alternative embodiment for the resistance element13. Resistance element 13a is provided, which comprises a reed relay 25operated by a coil 26. The reed relay 25 and coil 26 operate in the samemanner as the transistor 13, in that when there is voltage on the coil26 corresponding to a voltage on the base of the transistor 13, the reedrelay 25 is engaged and resistance is 0. When there is no signal on thecoil 26 from transistor 16, i.e., when transistor 16 is conducting andis shorted out, then reed relay 25 is disengaged and resistance isinfinite. Reed relay 25 and coil 26 provide a substantiallyinstantaneous transition from zero to infinite resistance which willcause the shape of the On and Off portions of the voltage on tube 11 tobe almost square.

It will be understood, of course, that control circuit 12 may beconnected to a whole series of tubes 11, each of which will contain anumber of distinct gas discharge elements which may be 1 through or maybe letters, or may be specific readouts. One side of all of theseelements will be connected in common to one side of resistance elements13 or 1301.

Obviously, many modifications and variations of the present inventionare possible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

-What is claimed is:

1. An apparatus for controlling the brightness of a light sourcecomprising:

a light source requiring a predetermined minimum voltage across it forconduction;

a first voltage source connected in circuit to said light source; andmeans for periodically interrupting the connection between said firstvoltage source and said light source, the ratio of connection time tononconnection time being variable, said means comprising:

a second voltage source;

a unijunction transistor having two base electrodes and an emitterelectrode, one base electrode coupled to said second voltage source;

a resistor-capacitor network connected in series between the other baseelectrode and the emitter electrode of said unijunction transistor;

a first variable resistance connected between said second voltage sourceand said emitter electrode for providing a variable charging current forsaid capacitor, said capacitor being discharged by said unijunctiontransistor upon reaching a prescribed voltage;

a first transistor;

a second variable resistance connected between the emitter electrode ofsaid unijunction transistor and the base of said first transistorthereby forming a voltage divider with said first transistor for varyingthe point of conduction thereof; and

means operatively connected to the collector of said first transistorand said light source for periodically interposing a substantiallyinfinite resistance between said light source and said first voltagesource as said resistor-capacitor network is charged and discharged.

2. An apparatus as recited in claim 1 wherein said means operativelyconnected to said first transistor comprises:

a second transistor having its conduction controlled by said firsttransistor and providing a substantially infinite resistance betweensaid light source and said first voltage source.

3. An apparatus as recited in claim 1 wherein said means operativelyconnected to said first transistor comprises:

a reed relay means for varying the resistance between said light sourceand said first voltage source in accordance with the conduction of saidfirst transistor.

References Cited by the Examiner UNITED STATES PATENTS 9/1965 Nielsen30788.5 12/1965 Phelps 30788.5

References Cited by the Applicant UNITED STATES PATENTS 3,063,00411/1962 Vic.

J. D. CRAIG, Assistant Examiner.

1. AN APPARATUS FOR CONTROLLING THE BRIGHTNESS OF A LIGHT SOURCECOMPRISING: A LIGHT SOURCE REQUIRING A PREDETERMINED MINIMUM VOLTAGEACROSS IT FOR CONDUCTION; A FIRST VOLTAGE SOURCE CONNECTED IN CIRCUIT TOSAID LIGHT SOURCE; AND MEANS FOR PERIODICALLY INTERRUPTING THECONNECTION BETWEEN SAID FIRST VOLTAGE SOURCE AND SAID LIGHT SOURCE, THERATIO OF CONNECTION TIME TO NONCONNECTION TIME BEING VARIABLE, SAIDMEANS COMPRISING: A SECOND VOLTAGE SOURCE; A UNIJUNCTION TRANSISTORHAVING TWO BASE ELECTRODES AND AN EMITTER ELECTRODE, ONE BASE ELECTRODECOUPLED TO SAID SECOND VOLTAGE SOURCE; A RESISTOR-CAPACITOR NETWORKCONNECTED IN SERIES BETWEEN THE OTHER BASE ELECTRODE AND THE EMITTERELECTRODE OF SAID UNIJUNCTION TRANSISTOR; A FIRST VARIABLE RESISTANCECONNECTED BETWEEN SAID SECOND VOLTAGE SOURCE AND SAID EMITTER ELECTRODEFOR PROVIDING A VARIABLE CHARGING CURRENT FOR SAID CAPACITOR, SAIDCAPACITOR BEING DISCHARGED BY SAID UNIJUNCTION TRANSISTOR UPON REACHINGA PRESCRIBED VOLTAGE; A FIRST TRANSISTOR; A SECOND VARIABLE RESISTANCECONNECTED BETWEEN THE EMITTER ELECTRODE OF SAID UNIJUNCTION TRANSISTORAND THE BASE OF SAID FIRST TRANSISTOR THEREBY FORMING A VOLTAGE DIVIDERWITH SAID FIRST TRANSISTOR FOR VARYING THE POINT OF CONDUCTION THEREOF;AND MEANS OPERATIVELY CONNECTED TO THE COLLECTOR OF SAID FIRSTTRANSISTOR AND SAID LIGHT SOURCE FOR PERIODICALLY INTERPOSING ASUBSTANTIALLY INFINITE RESISTANCE BETWEEN SAID LIGHT SOURCE AND SAIDFIRST VOLTAGE SOURCE AS SAID RESISTOR-CAPACITOR NETWORK IS CHARGED ANDDISCHARGED.